Flame spray pyrolysis13 currently established itself as a suitable method for the preparation of nanoparticles, most notably, oxides containing main group and transition metals41. It has rapidly evolved into a scalable process to oxide nanoparticles for catalyst preparation14,15 and industrial-scale flame-aerosol synthesis today produces megaton quantities of carbon, silica and titania. Experimentally, the flame spray reactors consist of a capillary surrounded by a narrow adjustable orifice (see FIG. 1). The precursor liquid is dispersed at the tip resulting in a well-defined spray. The surrounding methane/oxygen supporting flame ignites the spray and the flame converts the precursor to the corresponding materials.
For many applications nanoparticulate materials are desired. Such materials comprise calcium phosphates such as tricalciumphosphates but also apatites. Calcium phosphate biomaterials have attracted a tremendous interest in clinical medicine. Both hydroxyapatite (HAp or OHAp, Ca10(PO4)6(OH)2) and tricalcium phosphate (TCP, Ca3(PO4)2) exhibit excellent biocompatibility and osteoconductivity1,2. They are widely used for reparation of bony or periodontal defects, coating of metallic implants and bone space fillers. However, traditional methods (precipitation, sol-gel synthesis, hydrothermal method or solid-state reactions)1,3-5 suffer from a limited range of accessible materials and morphology. Wet-phase preparation generally requires time and cost intensive post treatments such as washing and drying. Solid-state reaction involves prolonged sintering and therefore results in low specific surface area powder. The rather dense materials display a lack of microporosity, reduce contact to the body fluid and hinder resorption in vivo.
Recently reported preparation methods comprise, plasma spraying6 and pulsed laser deposition7,8. They have resulted in advantageous coatings on implant surfaces. Moreover, amorphous calcium phosphates have shown to result in improved resorption properties9-11 and are promising materials for self-setting cements12 making them a most valuable target.
All these methods, however, have several drawbacks. They either lead to mixtures that can not be separated or only with considerable effort, and/or they lead to a too dense material, and/or they cannot be applied for bulk synthesis, and/or they are not usable in large scale production. Thus there is still a need for an improved production method allowing the production of pure materials, preferably also in large scale production, and an improved material obtainable by such method.